An RF cavity is used in a particle accelerator to accelerate a particle beam using an RF electromagnetic field. Development of fixed-field alternating gradient (FFAG) synchrotron technology has sparked interest in its use in various commercial machines. Upgrades of existing synchrotrons and improvements to other particle accelerator designs are also in need of RF cavities that fit into physically limited spaces of a practical machine and rapidly change frequency over a wide range. Thus, there is a need for RF cavities that are suitable for use in an accelerator whose acceptance depends on its functional capability as well as physical size.